Brushless DC motor and circuit for controlling the same

ABSTRACT

Disclosed is a multiphase brushless DC motor of a concentrated winding type having shunt connection, and a control circuit. The brushless DC motor includes a rotor made up of a permanent magnet with M number of poles, and a stator operating in K number of phases by means of windings wound on N number of teeth, wherein the plurality of windings having the same excitation phases wound on the teeth, are each maintained in shunt connection so as to improve driving torque and a rotational speed. A brushless DC motor includes a switching section having a plurality of upper switching devices and a plurality of lower switching devices connected with each other in series. Each of the windings is connected between a common joint between the upper and lower switching devices, and a common joint between a lower power supply voltage and a plurality of upper power supply voltages.

FIELD OF THE INVENTION

The present invention relates to a brushless DC (BLDC) motor. Moreparticularly, the present invention is directed to an independentmultiphase BLDC motor of a concentrated winding type having a shuntconnection, and a circuit for controlling such a motor.

BACKGROUND ART

Generally, brushless DC (BLDC) motors are designed so that a rotor ismade up of a permanent magnet and a stator is made up of an armaturewith a coil wound around a core. The BLDC motors are classified into asine-wave current driving type and a square-wave current driving typeaccording to the wave profile of current supplied to the armature. ABLDC motor is also called a permanent magnet motor, and has a broadapplication as a variable speed driving unit in the high performancedriving field, because the BLDC motor has a counter-electromotive forcewave profile of a trapezoidal shape, as well as a light weight, acompact size, a high efficiency, a small inertia and a simple drivingcircuit as compared with an induction motor having the same outputpower.

However, a BLDC motor generates pulsating or ripple torque and aresultant mechanical vibration due to cogging torque together withdriving torque for rotation, overlapping between phases, a spatialharmonic wave or the like, which lead to a reduced efficiency. Here,cogging torque is generated by interaction between stator slots androtor magnets. Cogging torque can be significantly reduced by skewingthe stator slots or rotor magnets by a pitch of one slot. Additionally,the pulsating or ripple components caused by mutual torque generated atthe regions where torques for each phase are overlapped can beconstrained by exciting a stator current sophisticatedly.

Typical BLDC motors have a plurality of windings, which function as anelectric circuit, inserted into a stator and/or a rotor. According tothe type of winding, BLDC motors are classified into a concentratedwinding type, in which independent coils are wound around each toothformed on the steel core, and a distributed winding type in which aplurality of windings are distributed into the corresponding teeth toform one phase. Of them, the concentrated winding type has been morewidely used, because the coil winding work is carried out on the outsideand the windings are then inserted around each tooth, thus it ispossible to accomplish easier automation than the distributed windingtype.

In addition, the conventional multiphase BLDC motors of the concentratedwinding type require current torque to be highly generated through ahigh input current for high-speed operation. Moreover, the conventionalmultiphase BLDC motors of the concentrated winding type that areconstructed in series connection are designed so that coils constitutingindividual phases are connected in series. Therefore, there aredrawbacks in that all BLDC motors have a high resistance value, thuslimiting the quantity of input current to a lower level, which makes itmore difficult to generate high torque and to operate at a high speed.

FIG. 1A is a schematic view depicting series connection of a BLDC motorhaving an outer rotor configuration according to the prior art FIG. 1Bis a schematic view depicting series connection of a BLDC motor havingan inner rotor configuration according to the prior art, and FIG. 1Cshows an equivalent circuit of an A-phase winding in case of the seriesconnection as in FIGS. 1A and 1B.

In a typical BLDC motor, the rotor is made up of a permanent magnet, andthe stator is designed so that a coil is wound around a continuousarrangement of teeth and slots. Here, when the rotor is arranged on theoutside of the stator, it is called an outer rotor structure. And, whenthe rotor is arranged on the inside of the stator, it is called an innerrotor structure.

Referring to FIGS. 1A and 1B, the stator 12 consists of nine teeth 13and nine slots 14. The nine teeth 13 are wound, as in the concentratedwinding type of stator, in such a manner that a coil 15 is wound aroundeach three teeth in a sequence of A phase, B phase and C phase, each ofwhich is connected in series. In this series connection, its equivalentcircuit is configured as shown in FIG. 1C so that three resistancecomponents R₁, R₂ and R₃ are connected with each other in series andresistance R_(—A) is relatively low. Therefore, there is a difficulty indriving the BLDC motor at a high speed due to a restriction of drivingcurrent. Specifically, a resistance loss is generated in proportion toI²R, and thus a coil making up each phase is connected in series and hasa high resistance value. Therefore, the multiphase BLDC motor of theconcentrated winding type constructed in series connection makes itdifficult to operate with a high efficiency due to a high resistanceloss. In addition, the multiphase BLDC motor of the concentrated windingtype constructed with series connections must be constructed for aplurality of coils making up each phase to be connected in series, sothat coil winding work must be carried out after all the cores arecompletely assembled. For this reason, the conventional BLDC motorhaving a series connection is not suitable for an automation process, sothat it has a low productivity. Moreover, even BLDC motors manufacturedthrough the same process have different properties.

As shown in FIG. 2, a switching circuit for driving the stator withthree phases, for example A-phase, B-phase and C-phase, requires fourswitching devices Q1 to Q4 per phase. Examples widely used for theswitching devices Q1 to Q4 are power semiconductors, such as IntegratedGate Bipolar Transistors (IGBTs), Metal-Oxide Semiconductor Field EffectTransistors (MOSFETs), Field Effect Transistors (FETs) and so forth.

Referring to FIG. 2, in an H-bridge 202 for the A-phase, each of theswitching devices Q1 to Q4 is controlled according to driving signalsA1+, A1−, A2+ and A2−. In an H-bridge 203 for the B-phase, each of theswitching devices Q1 to Q4 is controlled according to driving signalsB1+, B1−, B2+ and B2−. In an H-bridge 204 for the C-phase, each of theswitching devices Q1 to Q4 is controlled according to driving signalsC1+, C1−, C2+ and C2−.

In the BLDC motor constructed as the foregoing, gate driving signals forcontrolling the on/off state of each switching device Q1 to Q4 aregenerated. When the H-bridges are operated, Pulse Width Modulation (PWM)signals are applied to two of the four switching devices so as to turnon/off two switching devices in an alternate manner with respect to eachother. In other words, by setting the driving signals A1+ and A1− in theN-pole position of the rotor to be in a high state at the same time andthen turning the first and fourth switching devices Q1 and Q4 on, thecurrent path in the H-bridges runs in a counterclockwise crossdirection. In contrast, by setting the driving signals A2+ and A2− inthe S-pole position of the rotor to be in a high state at the same timeand then turning the second and third switching devices Q2 and Q3 on,the current path in the H-bridges runs in a clockwise cross direction. Adead time is set for not maintaining the driving signals A1+ and A1− andthe driving signals A2+ and A2− in a high state at the same time.

The switching circuit needs four switching devices so as to drive onephase. Therefore, the BLDC motor needs 4*K number of power switchingdevices to be driven for a certain magnitude and direction of phasecurrent, thus increasing production costs.

SUMMARY OF THE INVENTION

Therefore, the present invention has been made in view of theabove-mentioned problems, and it is an object of the present inventionto provide a brushless DC motor and a circuit for controlling the same,capable of allowing for a reduction of switching devices in a controlcircuit by independently controlling all the phases, each of which isindependently wound.

It is another object of the present invention to provide a brushless DCmotor with a shunt connection configuration, capable of performing highvelocity operation and high torque operation at a low voltage byproviding each phase with shunt or parallel connection.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, features and advantages of the presentinvention will become more apparent from the following detaileddescription when taken in conjunction with the accompanying drawings inwhich:

FIG. 1A is a schematic view depicting series connection of a BLDC motorhaving an outer rotor configuration according to the prior art;

FIG. 1B is a schematic view depicting series connection of a BLDC motorhaving an inner rotor configuration according to the prior art;

FIG. 1C shows an equivalent circuit of A-phase winding in the seriesconnections in FIGS. 1A and 1B;

FIG. 2 is a switching circuit diagram for supplying electric current toa stator of a multiphase BLDC motor according to the prior art;

FIG. 3 is a block diagram for controlling a BLDC motor of the presentinvention;

FIG. 4A is a schematic view depicting shunt connection of a BLDC motorhaving an outer rotor configuration according to the present invention;

FIG. 4B is a schematic view depicting shunt connection of a BLDC motorhaving an inner rotor configuration according to the present invention;

FIG. 4C shows an equivalent circuit of A-phase winding in case of theseries connection as in FIGS. 4A and 4B;

FIG. 5 is a switching circuit diagram according to the presentinvention;

FIG. 6 is a schematic view for illustrating an independent multiphasecontrol principle according to the present invention;

FIGS. 7A and 7B are schematic diagrams illustrating a procedure forcontrolling a BLDC motor in which the ratio of stator phase to rotorpole is 6:2;

FIGS. 8A and 8B are schematic diagrams illustrating a procedure forcontrolling a BLDC motor in which the ratio of stator phase to rotorpole is 6:4; and

FIGS. 9A and 9B are schematic diagrams illustrating a procedure forcontrolling a BLDC motor in which the ratio of stator phase to rotorpole is 6:6.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

In order to accomplish the first object, there is provided a brushlessDC motor, comprising: a rotor made up of a permanent magnet with Mnumber of poles; and a stator operating in K number of phases by meansof windings wound on N number of teeth, wherein the windings for the Kphases wound on the teeth are independently connected with each other,and are supplied with separate power supply voltages through a switchingsection so as to perform an independent control. A circuit forcontrolling a brushless DC motor of the present invention ischaracterized in that the switching section includes a plurality ofupper switching devices and a plurality of lower switching devicesconnected with each other in series; the power supply voltages comprisea lower power supply voltage for supplying one power supply voltage incommon through the lower switching devices and K number of upper powersupply voltages for supplying a plurality of power supply voltages inseparation through the upper switching devices; and each of theplurality of windings is independently connected between a common jointbetween the upper and lower switching devices, and a common jointbetween the lower power supply voltage and the plurality of upper powersupply voltages.

In order to accomplish the second object, there is provided a brushlessDC motor, comprising: a rotor made up of a permanent magnet with Mnumber of poles; and a stator including N number of teeth and slotswound by a plurality of windings, wherein the plurality of windingshaving the same excitation phases wound on the teeth are each maintainedin shunt connection so as to improve driving torque and rotationalspeed.

Hereinafter, the preferred embodiments of the present invention will bedescribed in detail with reference to the drawings.

As shown in FIG. 3, the configuration for controlling a brushless DC(BLDC) motor of the present invention comprises a DC power supply 302, aswitching section 304, a BLDC motor 306, an encoder 308, a controllingsection 310, and a gate driving section 312.

Referring to FIG. 3, the DC power supply 302 supplies various powersupply voltages, such as V+, Vo and V−, which the switching section 304requires. The switching section 304 includes a plurality of lower andupper switching devices, in which the lower switching devices arecapable of supplying a single power supply voltage (e.g., V+) in common,but the upper switching devices have to separately supply a plurality ofpower supply voltages. For example, in the case of k phases, the upperswitching devices require k+1 the power supply voltages.

The switching section 304 is implemented as two power switchingsemiconductor devices, which are connected with each other in series, asdescribed below. The switching section 304 applies each power supplyvoltage of the DC power supply 302 to a plurality of windings of thestator of the BLDC motor 306 according to gate driving signals.

The BLDC motor 306 according to the present invention is designed sothat a rotor is made up of a permanent magnet and a stator is made up ofan armature with a plurality of windings wound around a core. Asdescribed below, the BLDC motor 306 can be implemented as either anouter rotor structure or an inner rotor structure. The stator isconstructed so that at least one coil is wound around a continuousarrangement of teeth and slots. Then, the stator includes a connectionof a concentrated winding type, and generates a magnetic field accordingto electric current applied through the switching section 304, and thusinteracting with magnetic field of the rotator made up of the permanentmagnet to rotate the rotor. This BLDC motor 306 is typically called aN-phase M-pole BLDC, where N is the number of slots in the stator, and Mis the number of poles in the permanent magnet. Here, it is preferred todistinguish between an electrical phase K for controlling the BLDCmotor, an excitation phase J formed by winding around a plurality ofteeth and slots in series or in parallel so as to form a singleidentical electrical phase, and a mechanical phase N indicating thenumber of teeth and slots. However, “phase” as referred to in thepreferred embodiment of the present invention means the electrical phaseonly and specifically, as long as another type of phase is not referredto in particular.

When the BLDC motor rotates, a pole position is sensed by a hall sensoror an optical sensor (not shown), and simultaneously a rotational speedis sensed by the encoder 308. The sensed results are sent to thecontrolling section 310. This controlling section outputs controlsignals using command signals and sensor signals so as to rotate theBLDC motor at a desired speed.

The gate driving section 312 receives the control signals from thecontrolling section 310, and sequentially creates gate driving signalsfor driving power semiconductor devices constituting the switchingsection 304, and then applies the gate driving signals to respectivegates of the power semiconductor devices of the switching section.

FIG. 4A is a schematic view depicting a shunt connection of a BLDC motorhaving an outer rotor structure according to the present invention, FIG.4B is a schematic view depicting a shunt connection of a BLDC motorhaving an inner rotor structure according to the present invention, andFIG. 4C show an equivalent circuit of the shunt connection of A-phaseaccording to the present invention.

Referring to FIGS. 4A and 4B, a BLDC motor includes a rotor 404 and astator 406, which are arranged in a case 402. The stator 406 is designedso that windings 412 for each phase are connected in parallel and arewound around each tooth 408 and through each slot 410. This shuntconnection is an equivalent circuit in that resistances R₁, R₂ and R₃ ofeach winding are connected in parallel as shown in FIG. 4C, and thus itwill be seen that the total resistance R_(—A) becomes lower. Accordingto this configuration of the present invention, coils constituting eachphase of the BLDC motor are connected in parallel, so that a resistancevalue per phase can be maintained at a low level. Therefore, the BLDCmotor of the present invention has a low exothermic quantity caused byresistance loss, and a resulting high efficiency, thereby enabling theBLDC motor to accomplish a low voltage and high velocity operation, andto accomplish a low voltage and high torque operation.

In this BLDC motor, an optimal pole number ratio of the stator to therotor is 3*j:2*k, where j is number of excitation phases for supplyingelectric current to the windings of the stator at the same time, k is apositive integer, and 3*j is larger than 2*k. The optimal length of theteeth is 2π*j/(pole number of the rotor*N).

FIG. 5 is a circuit diagram of a switching section according to thepresent invention, which is for one phase. A switching section forapplying electric current to windings, which are wound around each toothof the stator in parallel, according to driving signals, includes twopower switching semiconductor devices Q1 and Q2, which are connectedwith each other in series.

Referring to FIG. 5, two power supply voltages, Vd+ and Vd−, aresupplied in series, and an upper switching device Q1 and a lowerswitching device Q2 are also connected with each other in series. Atleast one winding, which is wound around the teeth of the stator, isconnected between a common switching joint between the two switchingdevices Q1 and Q2, and a common voltage joint Vo, between two powersupply voltages.

In this circuit, when gate driving signals cause the upper switchingdevice Q1 to turn on and the lower switching device Q2 to turn off,electric current caused by the power supply voltage, Vd+, flows througha drain and a source of the upper switching device Q1 to the coil in adirection of the arrow {circle around (R)}. However, when gate drivingsignals cause the lower switching device Q2 to turn on and the upperswitching device Q1 to turn off, electric current flows from the commonswitching joint Vo through the coil (the direction of the arrow {circlearound (C)}) to a drain and a source of the lower switching device Q2.

This switching circuit according to the present invention is capable ofcontrolling the direction of electric current flowing to a coil by meansof two power only semiconductor devices for one phase, and thus thenumber of parts for use in a control circuit can be reduced as a whole,together with costs of production. Here, it should be noted that whenthe switching section is made up of K number of arms so as to drive Knumber of phases, lower switching devices of each arm are capable ofsupplying power using one power supply voltage Vd+, but upper switchingdevices require each separate power supply voltage, and the resultingK+1 number of powers as a whole. Additionally, windings of each phasemust be independent of each other, and thus they should be individuallyconnected to the corresponding power supply voltage.

FIG. 6 is a schematic view for illustrating an independent multiphasecontrol principle according to the present invention.

Referring to FIG. 6, for the present invention, three coils (threephases) A, B and C are independently wound with respect to each other,and are intended to supply power by controlling six switches SW1 to SW6.Specifically, in coil A, when a first switch SW1 turns on and a secondswitch SW2 turns off, electric current flows from Vd+ through the firstswitch SW1 to Vo. However, when the second switch SW2 turns on and thefirst switch SW1 turns off, electric current flows from Vo through thesecond switch SW2 to Vd−. In coil B, when a third switch SW3 turns onand a fourth switch SW4 turns off, electric current flows from Vd+through the third switch SW3 to Vo. By contrast when the fourth switchSW4 turns on and the third switch SW3 turns off, electric current flowsfrom Vo through the fourth switch SW4 to Vd−. Finally, in coil C, when afifth switch SW5 turns on and a sixth switch SW6 turns off, electriccurrent flows from Vd+ through the fifth switch SW5 to Vo. By contrast,when the sixth switch SW6 turns on and the fifth switch SW5 turns off,electric current flows from Vo through the sixth switch SW4 to Vd−.According to this configuration of the present invention, each phase isoperated with an independent power, and thus electric current of eachphase can be controlled independently.

FIG. 7 is a schematic diagram illustrating a procedure for controlling amotor independently, in which the motor includes a stator with six slots(teeth) and a rotor with two poles.

Referring to FIG. 7, a stator shown in FIG. 7A is designed so that sixwindings, indicated as A, B, C, A′, B′ and C′, are individually woundaround the corresponding teeth, and are each supplied with powersthrough the aforementioned switching circuit. A rotor arranged in thestator is made up of a permanent magnet having one N-pole and oneS-pole.

Here, it should be noted that in order to perform independent control,windings constituting each phase are irrelevant to whether they areconnected in series or in parallel, and windings for each phase must beconnected independently. As mentioned above, FIG. 1C shows the windingsfor A-phase, which are connected in series, FIG. 4C shows the windingsfor A-phase which are connected in parallel. For independent controlaccording to the present invention, it is of little importance whetherthe windings for A-phase are connected in series or in parallel, but theA-phase and the B-phase are independently connected with each other andmust be connected to separate power supply voltages.

In order to control the motor constructed as the forgoing, electriccurrent is sequentially applied for each winding in a sequence of A, B,C, A′, B′ and C′, as shown in FIG. 7B, so as to rotate the rotor. In thegraph, the transverse axis represents an angle while the rotor rotatesone turn (360 degrees), and the longitudinal axis represents eachwinding. It shows that when the corresponding winding is in a highstate, current is applied to generate torque. At the bottom of thegraph, the sum of the total torque is given.

As shown, torque is generated by the A-winding at a range from 0° to60°, by the B-winding at a range from 60° to 120°, and by the C-windingat a range from 120° to 180°. Also, torque is generated by theA′-winding at a range from 180° to 240°, by the B′-winding at a rangefrom 240° to 300°, and by the C′-winding at a range from 300° to 360°.

In the controlling section 310, driving signals for applying current toeach winding in this order are generated and then outputted to the gatedriving section 312. Then, in the gate driving section 312, thecorresponding switching device is turned on/off to apply current to thecorresponding winding, so that rotation of the BLDC motor can becontrolled.

FIG. 8 is a schematic diagram illustrating a procedure for controlling aBLDC motor according to the present invention, in which the BLDC motorincludes a stator with six slots (teeth) and a rotor with four poles.

Referring to FIG. 8, a stator shown in FIG. 8A is designed so that sixwindings indicated as A, B, C, A′, B′ and C′ are wound around thecorresponding teeth, and are each supplied with powers through theaforementioned switching circuit A rotor arranged in the stator is madeup of two permanent magnets, each of which consists of one N-pole andone S-pole.

In order to control the BLDC motor constructed as the forgoing, electriccurrent is sequentially applied to each winding in a sequence of A, B,C, A′, B′ and C′, as shown in FIG. 8B, so as to rotate the rotor. In thegraph, the transverse axis represents the angle of the rotor while therotor rotates one turn (360°), and the longitudinal axis represents eachwinding. It shows that when the corresponding winding is in a highstate, current is applied to generate torque. At the bottom of thegraph, the sum of the total torque is given.

As shown in FIG. 8, each phase is driven twice for a uniform interval.Torque is generated as current flows in the A-phase at both ranges from0° to 60° and from 180° to 240°, in the B-phase at both ranges from 60°to 120° and from 240° to 300°, and in the C-phase at both ranges from120° to 180° and from 300° to 360°. Similarly, torque is generated ascurrent flows in the A′-phase at both ranges from 90° to 150° and from270° to 330°, in the B′-phase at both ranges from 330° to 30° and from150° to 210°, and in the C′-phase at both ranges from 30° to 90° andfrom 210° to 270°.

FIG. 9 is a schematic diagram illustrating a procedure for controlling aBLDC motor according to the present invention, in which the motorincludes a stator with six slots (teeth) and a rotor with six poles.

Referring to FIG. 9, a stator shown in FIG. 9A is designed so that sixwindings, indicated as A, B, C, A′, B′ and C′, as shown in FIG. 9B, arewound around the corresponding teeth, and are supplied with individualpower through the aforementioned switching circuit A rotor arranged inthe stator is made up of three permanent magnets, each of which consistsof one N-pole and one S-pole.

As shown in FIG. 9, each phase is driven three times for a uniforminterval. Torque is generated as current flows in the A-phase at threeranges from 0° to 60°, from 120° to 180° and from 240° to 300°, in theB-phase at thee ranges from 60° to 120°, from 180° to 240° and from 300°to 360°, and in the C-phase, as the same as A-phase, at three rangesfrom 0° to 60°, from 120° to 180° and from 240° to 300°. Also, torque isgenerated as current flows in the A′-phase, as the same as B-phase, atthree ranges from 60° to 120°, from 180° to 240° and from 300° to 360°,in the B′-phase, as same as the C-phase, at three ranges from 0° to 60°,from 120° to 180° and from 240° to 300°, and in the C′-phase, as same asthe A′-phase, at both at three ranges from 60° to 120°, from 180° to240° and from 300° to 360°.

As can be seen from the foregoing, the driving circuit of a multiphasepermanent magnet motor of a concentrated winding type having K number ofindependent windings needs 2*K number of power switching devices so asto cause electric current inputted into each phase to adjust in acertain magnitude and direction, and needs K+1 number of independentpower supply voltages so as to drive the power switching devices.Therefore, the present invention is capable not only of accomplishing acost-effective control circuit, but also of performing an independentmultiphase control which makes it possible to freely control themagnitude and direction of the electric current in each phase, thusproviding a simple control circuit and control algorithm. In addition,it is possible to perform a low-voltage, high-speed operation and alow-voltage, high-torque operation by making stator windingsconstituting each separate phase to become parallel windings.

The BLDC motor of the present invention is constructed in a concentratedwinding type so that it can be expected to reduce torque ripple onlythrough adjustment in length of teeth, to have a high efficiency, and tofree from vibration. Moreover, the driving circuit is constructed usingat least switching devices, so that an improvement in reliability can beexpected.

1. In an apparatus in which a switching part allows a power of a DCpower part to be applied to a brushless DC motor comprising a rotorcomprised of a permanent magnet of M pole, a stator provided with Nnumber of teeth wound by coils and operated in K-phase, a controlcircuit of an independent multi-phase brushless DC motor beingcharacterized in that the stator is constructed such that windings ofK-phase wound on the teeth are independently connected, the DC powercomprising of a power Vd+, Vo that is voltage-divided by an uppercondenser (C) for applying power to upper switching devices and aresistance, and the Vd+, Vo corresponding to half of the DC power, and apower Vo, Vd− that is voltage-divided by a lower condenser for applyingpower to lower switching devices and a resistance, and corresponding tohalf of the DC power, K number of independent ground terminals Vocorresponding to common connection points of the cathode of the uppercondenser and the anode of the lower condenser are provided, theswitching part comprising an upper switching device of which one end isconnected to the power Vd+, Vo, and a lower switching device of whichone end is connected to the power Vo, Vd−, the upper switching deviceand the lower switching device being connected in series, the windingsof respective phases being independently connected between a serialconnection point of the two switching devices and the ground and theground terminal Vo, and when the upper switching device is turned on, acurrent by the voltage of Vd− flows to Vo via the turned-on upperswitching device and a corresponding winding, and when the lowerswitching device is turned on, the voltage of Vo flows to Vd− throughthe lower switching device.
 2. In an independent multiphase brushless DCmotor constructed to be independently controllable by a switching partand comprising a rotor comprised of a permanent magnet of M pole, astator having K-phase windings formed by coils wound on N number ofteeth and independently connected with each other, in which the windingsof independent exciting phase are respectively connected in parallel,when assuming that j is a number of excitation phases for applyingcurrent to the stator at the same time and k is an arbitrary naturalnumber, an optimal ratio of pole number of the rotator to pole number ofthe stator is 2*k to 3*j, and 3*j is greater than 2*k.